iStat Menus lets you monitor your system right from the menubar. Included are 8 menu extras that let you monitor every aspect of your system.
Some features:CPU -- Monitor cpu usage. 7 display modes, multiple core support.Memory -- Monitor memory usage. 4 display modes, page ins/outs and swap usage display.Disks -- Monitor disk usage and activity. 6 display modes, ability to hide disks you dont want to see.Network -- Monitor current and total bandwidth, peak bandwidth, IP addresses, and the ability to hide network connections you don't want to see.Temps -- Monitor the temperature of your Mac. 2 display modes, ability to hide sensors you don't want to see.Fans -- Monitor the fan speeds in your Mac. 2 display modes, ability to hide sensors you don't want to see.Bluetooth -- Control bluetooth status plus monitor the battery level of your Apple wireless keyboard or mouse.Date & Time -- Date and time in your menubar. World clock display lets you see the time in multiple locations around the world.

WHAT'S NEW Version 4.20:Added support for Late 2013 Mac Pro.Improved support for Late 2013 MacBook Pro.Improved support for Late 2013 iMac.Added support for monitoring multiple GPUs.Improved GPU monitoring.Improved Mavericks support.Fixed some display bugs in Network and Disk Activity history graphs.Fixed some calendar display bugs.Fixed a crash in the Sensors extra in OS X 10.9 Mavericks.Fixed a layout issue with long IPv6 addresses.Improved support for loading a large number of events in the Time extra.Improved error handling when calendar event database isn't available.Fixed some issues after migrating to a new Mac.Fixed some issues when iStat Menus is installed in /Applications/Utilities/